This invention relates to photographic objectives and more particularly to objectives with means for limiting variation of the angular field of view during focusing. Still more particularly it relates to zoom lenses employing such means.
In known lens systems, the front member facing the object is usually made movable to effect focusing. This is particularly so in zoom lenses because of the need to provide for zoom members, namely, a variator and a compensator with an object point (an image point of the focusing member) at a constant location independently of the distance from the lens to the object so as to avoid displacement of a sharp image plane while zooming.
This method of focusing when employed in an objective lens system and in particular a zoom lens system causes a serious problem by varying the angular field of view. In most photographic situations, a change in the angular field of view in the object space is perceptible during focusing which gives the photographer an unpleasant visual impression.
Various solutions to minimize the range of variation of the angular field during focusing have been proposed. According to P. Angenieux (British Pat. No. 975,160) and Takano (Japanese Patent Application Publication No. Sho 42-12103), the front member of the zoom lens is constructed from two lenses of negative and positive powers of which the frontmost negative lens only is made movable for focusing purposes for minimizing the angular field variation. According to G. H. Cook et al. (U.S. Pat. No. 3,682,534), the front member is divided into two parts and the front part is stationary during the focusing movement of the rear part. This reduces the angular field variation. According to Takeshi et al (U.S. Pat. No. 4,099,845), the front member comprises negative, positive and positive lens groups of which only the second is moved toward the rear as focusing is effected down to shorter object distances so as to minimize the angular field of view. The principal aims of these focusing lens designs were to reduce the diameter of the front member and to shorten the object distance for close up adjustment. The minimization of the variation of the angular field may be considered as being secondary. What P. Angenieux, Takano, G. H. Cook et al and Takeshi et al have done in respect to the change in the angular field is, therefore, the mere reduction of the range of variation, and much variation remains.
Unlike these focusing methods, Tanaka has achieved the removal of the variation of the angular field by moving two lens groups which are arranged at the front of an optical lens system in a differential relation to each other such that the rear principal point of the lens system remains stationary in position during focusing, as disclosed in West-German Utility Model No. 7,908,085. However, it has been found that Tanaka's method, though suited for application in the lens system in which the rear principal point of the entire system is near the exit pupil, gives rise to errors when applied to a lens system in which the rear principal point and the exit pupil are substantially separated from each other.